This invention relates to an equalizer circuit of the kind used at the receiving end of a fiber optic system or other digital transmission system to remove distortion introduced by the transmission medium. In particular, the circuit of the present invention is suitable for use in an adaptive equalizer, as well as a manual one.
In digital transmission systems, filtering actions of the transmission medium cause distortion of the transmitted digital pulses such that pulses at the receiver may be spread over more than one baud interval. This intersymbol interference causes the "eye" opening exhibited by pulses at the receiver to close, beginnning in the corners and progressing toward the center. The decision as to whether received data is a one or a zero can be made at the center of the baud interval, but as this portion of the eye begins to close, receiver sensitivity is reduced. It is the role of an equalizer to reduce intersymbol interference as much as possible so as to restore the receiver sensitivity.
In fiber optic systems, not all fibers produce exactly the same filtering of the transmitted pulses. Accordingly, it is desirable to provide an adaptive equalizer which automatically adjusts its equalization to an individual fiber.